When a tablespace(tbs) is in 'backup mode' the headers of the datafiles in that tbs are freezed and no entry is made to the datafile but to the redo logs instead. So, what ever changes are made to this datafile the entries are placed in the redo log, this can generate a lot of redo logs and overhead to LGWR. For this reason, u should try to minimize the time between 'begin backup' and 'end backup'. This can be done by giving these commands at the datafile level and instead of the tablespace level.

Correspondingly, the size & number of u'r redologs should be sufficient for u'r normal db operations during online backup.
The optimal would be say 3 groups with 2 members each of size say 10MB. This ofcouse, depends on the nature of u'r transactions.

Hope this helps.

With regards.